The present invention relates to a power supply circuit.
In the prior art, portable electronic devices, such as MD players and the like, are operated by a power supply, such as a rechargeable battery. Recent portable devices are capable of transferring data to/from external devices, such as a personal computer (PC) and the like, through a communication cable complying with the universal serial bus (USB) standard, the Institute of Electrical and Electronics Engineers (IEEE) 1394 standard, or the like.
Interface standards, such as the USB standard, specify a communication cable that is capable of supplying power. An external device is constructed so that it can supply the specified power. Therefore, many portable electronic devices operate on power (bus power) supplied from an external device through a communication cable when the portable device is connected to the external device and operate on battery power from, for example, a rechargeable battery, when the portable device is not connected to the external device.
Japanese Laid-Open Patent Publication No. 2005-124317 describes a bus powered device in which power is supplied from an external device through a communication cable or from a battery. The power is selectively supplied to a CPU and power load by a switch device controlled by the CPU. Publication No. 2005-124317, the CPU that controls the switch device is also operated by the single power selected from the two power sources. This may disable control of the switch device. For example, when the CPU is operated by the power supplied from an external device through the communication cable, unplugging of the communication cable would interrupt the power from the external device. Thus, the supply of power to the CPU would stop, and the switch device cannot be switched so as to continue operation. In the same manner, the switch device cannot be controlled when power is not supplied to the CPU, such as during startup.
Therefore, the anodes of two diodes may respectively be supplied with power from a battery and from a communication cable. In this case, the cathodes of the two diodes are connected to each other so that power is supplied to the CPU and power load from a node between the cathodes. In this configuration, however, a voltage drop may occur between the anode and cathode of the diodes, and the cutting of one power supply cannot be ensured. This would be inefficient from the viewpoint of power consumption.